1. Field of the Invention
The present invention relates to a pushbutton switch key sheet for use in an operating portion of an apparatus such as a mobile phone, a PDA, a car navigation apparatus, or a car audio apparatus.
2. Related Background Art
Electronic/electric apparatuses for personal use have been reduced in size and thickness. Further, they have come to be endowed with various additional functions for enhancement in convenience for the user. However, for the better commercial value, in addition to the abundance in functions, it is important for this kind of product to have a unique and novel design. With regard to the mobile phone and the like, there is a tough competition for a design of a pushbutton switch key sheet forming a surface portion.
In a conventional example of such a pushbutton switch key sheet, a single operation plate having a plurality of depressing operation portions is exposed through an operational opening of a casing having no frame. Despite the absence of a frame, this key sheet does not suffer distortion or deflection. However, since the depressing operation portions are separated from each other by dividing grooves formed in the operation plate, nails or fingertips may get deep into the dividing grooves of the frame, and the nails or the fingertips may be injured by edges of the dividing grooves. To avoid this problem, the present applicant has proposed filling the dividing grooves with a rubber-like elastic material in JP 2006-156333 A (the specification of Japanese Patent Application No. 2005-133414).
However, the rubber-like elastic material filling the dividing grooves is subject to wear; as depressing operation is repeated, the rubber-like elastic material is gradually worn out by the fingertips, and there is a fear of the dividing grooves becoming gradually deeper. Then, nails or fingertips are allowed to enter the dividing grooves to suffer injury by edges of the dividing grooves. To prevent wear of the rubber-like elastic material filling the dividing grooves, a method is available according to which the operating surface of the operation plate is covered with a resin film. However, covering the operating surface with a resin film will lead to a kind of operational connection between adjacent depressing operation portions, and make it rather difficult to recognize the division between the depressing operation portions by the touch of the fingertips, resulting in erroneous input.